According to RT News, if spent fuel pool #4 drains or falls over due to a large earthquake, the cesium released from this pool and the others that would quickly follow would be 85 times the amount of radiation released by Chernobyl.

If this spent fuel pool/building collapses, it means the end of Japan. It very well may mean massive radioactive contamination of the whole planet, possibly worse,[/b] if it results in a cascade failure of other spent fuel pools at Fukushima.